Field of the Invention
The invention concerns a method for acquiring magnetic resonance data of a slice package composed of multiple measured slices as a target volume with a magnetic resonance scanner using a measuring sequence, wherein the measuring sequence includes, prior to each scan of one of the measured slices, a preparation pulse associated with the measured slice for signal suppression of one type of saturated molecule, said preparation pulse acting on the entire target volume, wherein a pulse parameter of the preparation pulse is chosen for a measured slice group, composed of at least one measured slice, as a function of resonance information of the contiguous partial volume covered by the measured slice group. In addition, the invention concerns a magnetic resonance apparatus and an electronically readable data carrier for implementing such a method.
Description of the Prior Art
Saturation techniques, in particular fat saturation techniques, are extensively known in magnetic resonance imaging and are frequently employed in clinical imaging. Such fat saturation techniques mostly exploit the fact that the resonance frequencies of fat and water are slightly different, this being known as chemical shift. However, such chemical shifts also exist with respect to other types of molecules, such that saturation techniques that are intended to separate types of molecule other than fat and water from one another can also be used in imaging, for example to suppress silicone. In this case a preparation pulse, which can also be called a saturation pulse, is emitted, and excites only the spin of the type of molecule whose signal is to be suppressed, such that the magnetization thereof is saturated and not affected by other, specific excitation pulses, which then act on the other type of molecule, in other words the type of target molecule from which the magnetic resonance signals are to be received (acquired), Such a measurement sequence is shown in FIG. 7.
Fat saturation techniques or other saturation techniques respond extremely sensitively to inhomogeneous spatial distributions of the static magnetic field, in other words of the main magnetic field (B0). In the regions in which these field inhomogeneities occur, two undesired effects can be observed, namely incomplete fat saturation and unwanted water saturation. The reason for this is mainly that the fat saturation requires frequency-selective preparation pulses that act on the entire volume of the examination object, at least the entire target volume to be acquired in the form of multiple slices, where strong field inhomogeneities can occur, particularly at higher magnetic field strengths. The strength and the spatial distribution of such field inhomogeneities depend on the anatomical region. For example, particularly strong inhomogeneities occur in the thorax, since air is present there in the lungs, which has a very different magnetic susceptibility compared to biological tissue.
The effect of the fat saturation pulse or general preparation pulse is based on the specific choice of its pulse parameters, in particular the pulse frequency and the pulse bandwidth. Magnetic field inhomogeneities mean that the fat and water peaks in the frequency spectrum, if whole partial volumes such as measured slices are considered, are widened, such that the frequency range between the two peaks becomes a critical region that may contain elements of both types of molecules. The result here can be limited to fat saturation or unwanted water saturation.
If preparation pulses are used to suppress signals of a type of saturation molecule, for example from fat, their pulse frequency or nominal frequency usually differs precisely by the chemical shift to the type of target molecule from the subsequent excitation pulse which is to excite the non-saturated spin of the type of target molecule, in order to measure the decay of the excitation, as known, as magnetic resonance signals and to acquire corresponding magnetic resonance data. If only extremely narrow regions are acquired, the problem of field inhomogeneities can be suppressed, by using different frequency shifts and pulse frequencies for the preparation pulse and the excitation pulse as a function of the partial volume acquired, in particular specifically as a function of the anatomical region. Many imaging techniques, however, are aimed at the acquisition of larger target volumes within the human body, which then are usually acquired as slice packages of a number of measured slices captured one after the other. The larger the acquired target volume, the larger also are the spatial variations in the basic magnetic field, so that severe problems can occur.
In the subsequently published German patent application DE 10 2016 202 400.0 it was proposed in this respect, in order to achieve improved chemical selectivity in the saturation of the magnetization of particular types of molecule, to adjust the coil currents of shim coils, which are used to reduce a local inhomogeneity in the basic magnetic field, and/or at least one pulse parameter of the preparation pulse as a function of the position in the target volume of the measured slice currently to be acquired. It is consequently possible, when emitting the preparation pulse, to take into consideration the position of the target volume at which the measured slice, for which magnetization data is to be captured immediately after the transmission of the preparation pulse, is located. Because of the known position of the measured slice, the chemical selectivity of the preparation pulse can be optimized locally for the respective measured slice to be examined or for a partial volume of the measured volume which includes said measured slice. The at least one pulse parameter can be a pulse frequency and/or a spectral composition and/or a pulse amplitude of the preparation pulse and/or a relative transmit amplitude for different antennas via which the preparation pulse is emitted.
The pulse frequency describes the frequency at which the spectral distribution of the preparation pulse is at its maximum, which also applies for the following description regardless of the subsequent printed publication. By adjusting the spectral composition and/or the pulse frequency it is possible in particular to balance mistunings of the resonance frequency of the type of molecule to be excited based on inhomogeneities in the main magnetic field. However, this means that the pulse parameters are adjusted in accordance with DE 10 2016 202 400.0 if need be as a function of an absolute B0 field map in relation to the type of target molecule. An improvement can in fact be achieved in this way, further potential nevertheless existing for an improvement in the saturation behavior and thus the quality of the magnetic resonance data to be acquired.